1. Field of the Invention
The present invention relates to scanning circuits of cathode ray tubes (CRTs) and more particularly to such circuits with a low power consumption at standby.
2. Discussion of the Related Art
FIG. 1 shows an exemplary CRT vertical scanning circuit together with control and supply circuits.
A power supply comprises a transformer 1 with a primary winding L1 connected to an AC supply source and two secondary windings L2+ and L2−. The secondary winding L2+ is connected between the ground and the anode of a diode D+. A storage capacitor C+ is connected between the cathode of the diode D+ and the ground. The secondary winding L2− is connected between the ground and the cathode of a diode D−. A storage capacitor C− is connected between the anode of the diode D− and the ground. In operation, the cathode of the diode D+ is at a positive voltage, for example +12V, and is connected to a terminal of a switch S. The other terminal of the switch S is connected to a terminal Tup. The anode of the diode D− is at a negative voltage, for example −12V, and is connected to a terminal Tdown.
A typical vertical scanning circuit 2 comprises a differential amplifier 3 providing a control signal c to a power amplifier 4. The output of the power amplifier 4 is connected to a first terminal of a deflection coil Ly. The second terminal of the deflection coil Ly is linked to the ground through a resistor R. Amplifiers 3, 4 are supplied by connections to terminals Tdown and Tup. The differential amplifier 3 receives negative and positive input signals E+ and E− from a control circuit 6 supplied by connections to terminal Tup and to the ground. The intermediate node Z between the deflection coil Ly and the resistor R is connected to the control circuit 6.
In operation, the switch S is closed. During the screen scanning, the deflection coil Ly current decreases linearly so that each scanned line is under the former one. Between two screen scannings, the current in deflection coil Ly increases very quickly, so that the spot is moved from the last line to the first one. The power amplifier 4 is in this example an inverting amplifier. When the difference voltage between input signals E+ and E− is positive, the current through the deflection coil Ly flows from the ground to terminal Tdown through “low-side” current paths of the power amplifier 4. When the difference voltage between input signals E+ and E− is negative, the current through the deflection coil Ly goes from terminal Tup to the ground through “high-side” current paths of the power amplifier 4.
So as to make sure that the current through the deflection coil Ly is correctly set by the vertical scanning circuit 2, the control circuit 6 measures the current in the deflection coil Ly by analyzing the voltage of the node Z. The negative input signal E− produced by the control circuit 6 fluctuates according to the drift of the measured current compared to the expected current in the deflection coil Ly. The positive input signal E+ acts as a control signal. If the measured current is too high, the voltage difference between the positive input signal E+ and the negative input signal E− is lowered so as to decrease the current in the deflection coil and conversely.
In standby mode, power consumption must be minimized. The switch S is open to save power. The terminal Tup is no longer powered. The voltage of terminal Tup decreases as a current is drawn by the amplifiers 3, 4. However, when the control circuit 6 is an integrated circuit, there are diodes 7 whose anodes are connected to the ground and whose cathodes are connected to terminal Tup. Therefore it remains a current path for discharging capacitor C− and the active elements remain supplied substantially at one half of the normal supply voltage. This causes a low power consumption that is generally considered tolerable when compared to the drawbacks of using a second switch for disconnecting capacitor C−.
However, the inventor has noted that in about one vertical scanning circuit out of two, the power consumption in the standby state was much higher than expected.
Consequently, the purpose of this invention is to present a scanning circuit such that its power consumption is always minimal in standby mode.